linux-kernel - Re: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write lock in exit

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Message-ID: <YbDUnkmQP3nxd5bv@dhcp22.suse.cz>
Date:   Wed, 8 Dec 2021 16:51:58 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        rientjes@...gle.com, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write
 lock in exit_mmap

On Wed 08-12-21 15:01:24, Matthew Wilcox wrote:
> On Tue, Dec 07, 2021 at 03:08:19PM -0800, Suren Baghdasaryan wrote:
> > > >         /**
> > > >          * @close: Called when the VMA is being removed from the MM.
> > > >          * Context: Caller holds mmap_lock.
> > 
> > BTW, is the caller always required to hold mmap_lock for write or it
> > *might* hold it?
> 
> __do_munmap() might hold it for read, thanks to:
> 
>         if (downgrade)
>                 mmap_write_downgrade(mm);
> 
> Should probably say:
> 
> 	* Context: User context.  May sleep.  Caller holds mmap_lock.
> 
> I don't think we should burden the implementor of the vm_ops with the
> knowledge that the VM chooses to not hold the mmap_lock under certain
> circumstances when it doesn't matter whether it's holding the mmap_lock
> or not.

If we document it like that some code might depend on that lock to be
held. I think we only want to document that the holder itself is not
allowed to take mmap sem or a depending lock.

-- 
Michal Hocko
SUSE Labs